Joining of aluminum by brazing is a well known process due to the strong and uniform joints that can be produced between aluminum parts of varying shapes and types. A commercially important brazing process today is the Controlled Atmosphere Brazing process hereinafter the CAB process. As the name implies, the CAB process is performed in a furnace with a controlled atmosphere having low oxygen and moisture content to minimize oxidation of aluminum at elevated temperatures. In the CAB process, aluminum parts to be joined are formed, sometimes cleaned, fluxed and then brazed at temperatures close to 600° C. The CAB process requires melting of a filler metal, typically a 4XXX series (Al—Si) aluminum alloy that has a lower melting temperature than the aluminum parts that are being joined. The filler metal can be added as foil between the aluminum parts being joined, as a powder in a paste placed near joint locations, or it can be present as an integral part of the pieces being joined if the pieces are fabricated from a clad product commonly known as an aluminum brazing sheet. The role of flux, which melts prior to the filler metal, is to lift or dissolve the oxide layer on the aluminum parts and to further protect the underlying metal preventing re-oxidation. One example of a family of fluxes suitable for the CAB brazing is inorganic fluoride fluxes, such as potassium fluoro-aluminates. One widely used commercial flux of this type is the Nocolok® family of fluxes. Nocolok is a registered trademark of Alcan Aluminum Ltd of Canada.
It remains a challenge today to generate high strength in CAB brazed aluminum parts, wherein high strength generally refers to ultimate tensile strength (UTS) values of 190 MPa or greater. Recrystallization of an aluminum core alloy removes any prior strengthening from deformation, and most solute additions to the aluminum core alloys offer only modest strength increases. It is well known, however, that combinations of solutes in aluminum can result in precipitation hardening under certain conditions of concentration and thermal history. One particularly effective pair of solutes is Magnesium and Silicon, which can combine to form very small precipitates that strengthen the aluminum parts. This is commonly called age-hardening. When the precipitation occurs at room temperature it is called “natural aging” and at elevated temperatures “artificial aging”. Under the right conditions Cu can also participate in age hardening reactions with Mg and Si by forming Al—Cu—Mg or Al—Cu—Mg—Si compounds as small strengthening precipitates.
While Si is a common element in aluminum braze alloys and is often present in the AA3XXX alloys commonly used as the core layer of aluminum brazing sheet, Mg is often restricted in brazing alloys used in the CAB process. This is due to the known detrimental effect Mg has on Nocolok® type fluxes. Mg interacts with the flux in a way that has a negative impact on brazing performance. Nocolok flux has a low solvating capacity for MgO and the flux reacts with Mg and MgO to form magnesium fluorides which raise the melting point of the flux and reduce its activity. If Mg is present in the core layer in high enough concentrations (sometimes even as little as 0.1% can be detrimental) it can diffuse into a braze liner during the CAB process to interact with the flux. One strategy of taking advantage of Mg, while still maintaining good brazing performance in the CAB process, has been to incorporate Mg into a liner on the opposite side of the core from the braze liner. This liner is called a water-side liner or sacrificial layer because it will contact the coolant in a engine cooling circuit. Further, with a proper choice of composition the sacrificial layer can also provide cathodic protection to the underlying core alloy and thus help minimize the severity of internal corrosion attack on the tube in service. This strategy of incorporating Mg in the sacrificial layer is very effective for welded tubes where the edges of tubestock strip get seam welded together. The thick core of the tubestock provides a sufficient diffusion barrier for the Mg so that it does not reach the filler metal or flux during the brazing operation.
Different tube designs, however, can be used to provide tubes of additional strength. These designs call for folding tubestock into a configuration that allows for the tube to have a central web along the mid-depth of the tube, separating the tube into two parallel flow channels. These are sometimes referred to as B-tube configurations because of their appearance in crossection. However, for B-tube configurations, the use of a high Mg sacrificial layer will interfere with good braze joint formation at locations where the braze filler metal needs to wet the internal sacrificial layer.
To solve this problem, U.S. Pat. No. 6,555,251 to Kilmer isolated the Mg-bearing core from both the braze clad layer and the sacrificial layer by using a four-layer tubestock construction. In this construction a Mg-bearing core was bounded on both surfaces by Mg-free interliners. A key feature of this later invention was that one or both of the interliners was higher in Si than the core alloy. A second key feature of that design was the use of modest Cu levels (up to about 0.3%) in the first interliner (the layer between the braze layer and the core) believed to establish a corrosion potential gradient that would be favorable for external corrosion resistance of the tube. Kilmer reported post-braze Ultimate Tensile Strength (UTS) values approaching 150 MPa after 30 days of natural aging, and UTS as high as 210 MPa after significant time at elevated temperature (30 days at 90° C.).
In light of the above, there remains a need in the art for tubestock materials with excellent braze-ability, that achieve higher levels of post braze strength with shorter aging times, preferably at room temperature, and that further exhibit good external and internal corrosion characteristics that can further be used in folded-tube configurations including but not limited to B-tube configurations.